Effect of interfacial transition zone and aggregates on the time-dependent behavior of mortar and concrete

International audienceConcrete has a high degree of heterogeneity. About 75 % of the volume is occupied by aggregates. Even if most properties of concrete come from the cement paste (shrinkage, creep …), aggregates modify in a large extent the properties of concrete. In this paper, the effect of sand grains, on the cracking process of mortars, is numerically studied. Digital picture of mortar are used for finite element simulations to study the drying shrinkage. The obtained cracking pattern is compared to the observed one in the literature, which shows good agreements

Prediction of elastic properties of cement pastes at early ages

Cementitious materials are known to be sensitive to cracking at early ages. During the first days which follow the contact between water and cement, the system is continuously evolving, as its mechanical characteristics follow a rapid rate of change and the material is prone to cracking. One of the parameters that highly influence the behavior of the material at early ages is the Young's modulus. Analytical calculations, based on existing homogenization models and finite element calculations, applied on a discrete generated microstructure, are first considered in order to predict the elastic properties of the material. As long as the cohesive role played by the hydrates is not taken into account, results at early age remain inaccurate, especially for low watercement ratios. The need of modeling an intrinsic characteristic of cementitious materials setting arises. An approach, based on percolation and on the so-called burning algorithm, which takes into account explicitly the bonding role of hydrates and reveals a degree of hydration threshold below which the rigidity of the material is negligible, is therefore proposed. The evolution of the elastic characteristics is obtained by applying the previous computation methods to the percolation cluster given by the burning algorithm

Calcul de perméabilité en milieu fissuré : approche méso-macro

International audienceIn this paper, a sequential multi-scale framework to solve mass (air or water) transfer problems is described. Numerical results are checked against mechanical and permeation experimental datas from a reinforced concrete specimen under tensile load designed by C. Desmettre and J.P. CharronCe papier présente une approche multi-échelle séquentielle permettant de résoudre des problèmes de transfert de masse (air ou eau). Les résultats numériques sont confrontés à des données expérimentales obtenues par C. Desmettre et J.P. Charron [DES 11]. Ces derniers ont mesuré le débit traversant un tirant en béton armé sous différents paliers de chargement

Experimental investigation of the variability of concrete durability properties

One of the main objectives of the APPLET project was to quantify the variability of concrete properties to allow for a probabilistic performance-based approach regarding the service lifetime prediction of concrete structures. The characterization of concrete variability was the subject of an experimental program which included a significant number of tests allowing the characterization of durability indicators or performance tests. Two construction sites were selected from which concrete specimens were periodically taken and tested by the different project partners. The obtained results (mechanical behavior, chloride migration, accelerated carbonation, gas permeability, desorption isotherms, porosity) are discussed and a statistical analysis was performed to characterize these results through appropriate probability density functions

MODELISATION DES DEFORMATIONS DIFFEREES DU BETON SOUS SOLLICITATIONS BIAXIALES. APPLICATION AUX ENCEINTES DE CONFINEMENT DE BATIMENTS REACTEURS DES CENTRALES NUCLEAIRES

The prediction of delayed strains is of crucial importance for durability and long-term serviceability of concrete structures (bridges, containment vessels of nuclear power plants, etc.). Indeed, creep and shrinkage cause cracking, losses of pre-stress and redistribution of stresses, and also, rarely, the ruin of the structure.The objective of this work is to develop numerical tools, able to predict the long-term behavior of concrete structures. Thus, a new hydro mechanical model is developed, including the description of drying, shrinkage, creep and cracking phenomena for concrete as a non-saturated porous medium. The modeling of drying shrinkage is based on an unified approach of creep and shrinkage. Basic and drying creep models are based on relevant chemo-physical mechanisms, which occur at different scales of the cement paste. The basic creep is explicitly related to the micro-diffusion of the adsorbed water between interhydrates and intrahydrates and the capillary pores, and the sliding of the C-S-H gel at the nano-porosity level. The drying creep is induced by the micro-diffusion of the adsorbed water at different scales of the porosity, under the simultaneous effects of drying and mechanical loadings. Drying shrinkage is, therefore, assumed to result from the elastic and delayed response of the solid skeleton, submitted to both capillary and disjoining pressures. Furthermore, the cracking behavior of concrete is described by an orthotropic elastoplastic damage model. The coupling between all these phenomena is performed by using effective stresses which account for both external applied stresses and pore pressures.This model has been incorporated into a finite element code. The analysis of the long-term behavior is also performed on concrete specimens and prestressed concrete structures submitted to simultaneous drying and mechanical loadings.La prédiction des déformations différées est d'une très grande importance pour l'étude de la durabilité et de l'aptitude au fonctionnement à long terme des structures en béton (ponts, enceintes de confinement de bâtiments réacteurs des centrales nucléaires, etc.). En effet, elles peuvent être à l'origine de la fissuration, de pertes de précontrainte, d'une redistribution des contraintes et même, plus rarement, de la ruine de l'ouvrage.L'objectif de ce travail est alors de développer des outils de calcul numérique, capable de prédire le comportement différé de structures en béton. Pour cela, un nouveau modèle hydromécanique du béton est développé, intégrant la description des phénomènes de séchage, de retrait, de fluage et de fissuration. La modélisation du retrait de dessiccation est basée sur une approche unifiée du fluage et du retrait. Les modèles de fluage propre et de fluage de dessiccation sont basés sur des mécanismes physico-chimiques plausibles, se produisant à différentes échelles d'observation de la pâte de ciment. Le modèle de fluage propre est associé à la micro-diffusion de l'eau adsorbée entre la porosité interhydrates et intrahydrates et la porosité capillaire, et au glissement des feuillets de C-S-H à l'échelle des nanopores. Le fluage de dessiccation est induit par la micro-diffusion de l'eau adsorbée à différentes échelles de porosité sous l'effet d'une sollicitation mécanique et hydrique combinée. Le retrait de dessiccation résulte, en effet, de la déformation élastique et différée du squelette solide, sous les effets de la pression capillaire et de la pression de disjonction. Le comportement mécanique du béton fissuré est modélisé en utilisant le formalisme de l'élastoplasticité endommageable orthotrope. La combinaison de ces phénomènes est effectuée dans le cadre de la mécanique des milieux poreux non saturés, en s'appuyant sur le concept des contraintes effectives.Ce modèle a été incorporé dans un code de calcul aux éléments finis. L'analyse du comportement différé d'éprouvettes et de structures en béton et en béton précontraint, soumises à des sollicitations hydriques et mécaniques combinées, est alors présentée

Modélisation des déformations différées du béton sous sollicitations biaxiales (application aux enceintes de confinement de bâtiments réacteurs des centrales nucléaires)

La prédiction des déformations différées est d une très grande importance pour l étude de la durabilité et de l aptitude au fonctionnement à long terme des structures en béton (ponts, enceintes de confinement de bâtiments réacteurs des centrales nucléaires, etc.). En effet, elles peuvent être à l origine de la fissuration, de pertes de précontrainte, d une redistribution des contraintes et même, plus rarement, de la ruine de l ouvrage. L objectif de ce travail est alors de développer des outils de calcul numérique, capable de prédire le comportement différé de structures en béton. Pour cela, un nouveau modèle hydromécanique du béton est développé, intégrant la description des phénomènes de séchage, de retrait, de fluage et de fissuration. La modélisation du retrait de dessiccation est basée sur une approche unifiée du fluage et du retrait. Les modèles de fluage propre et de fluage de dessiccation sont basés sur des mécanismes hysico-chimiques plausibles, se produisant à différentes échelles d observation de la pâte de ciment. Le modèle de fluage propre est associé à la micro-diffusion de l eau adsorbée entre la porosité interhydrates et intrahydrates et la porosité capillaire, et au glissement des feuillets de C-S-H à l échelle des nanopores. Le fluage de dessiccation est induit par la micro-diffusion de l eau adsorbée à différentes échelles de porosité sous l effet d une sollicitation mécanique et hydrique combinée. Le retrait de dessiccation résulte, en effet, de la déformation élastique et différée du squelette solide, sous les effets de la pression capillaire et de la pression de disjonction. Le comportement mécanique du béton fissuré est modélisé en utilisant le formalisme de l élastoplasticité endommageable orthotrope. La combinaison de ces phénomènes est effectuée dans le cadre de la mécanique des milieux poreux non saturés, en s appuyant sur le concept des contraintes effectives. Ce modèle a été incorporé dans un code de calcul aux éléments finis. L analyse du comportement différé d éprouvettes et de structures en béton et en béton précontraint, soumises à des sollicitations hydriques et mécaniques combinées, est alors présentéeThe prediction of delayed strains is of crucial importance for durability and long-term serviceability of concrete structures (bridges, containment vessels of nuclear power plants, etc.). Indeed, creep and shrinkage cause cracking, losses of pre-stress and redistribution of stresses, and also, rarely, the ruin of the structure. The objective of this work is to develop numerical tools, able to predict the long-term behavior of concrete structures. Thus, a new hydro mechanical model is developed, including the description of drying, shrinkage, creep and cracking phenomena for concrete as a nonsaturated porous medium. The modeling of drying shrinkage is based on an unified approach of creep and shrinkage. Basic and drying creep models are based on relevant chemo-physical mechanisms, which occur at different scales of the cement paste. The basic creep is explicitly related to the micro-diffusion of the adsorbed water between interhydrates and intrahydrates and the capillary pores, and the sliding of the C-S-H gel at the nano-porosity level. The drying creep is induced by the micro-diffusion of the adsorbed water at different scales of the porosity, under the simultaneous effects of drying and mechanical loadings. Drying shrinkage is, therefore, assumed to result from the elastic and delayed response of the solid skeleton, submitted to both capillary and disjoining pressures. Furthermore, the cracking behavior of concrete is described by an orthotropic elastoplastic damage model. The coupling between all these phenomena is performed by using effective stresses which account for both external applied stresses and pore pressuresPARIS-EST Marne-la-Vallee-BU (774682101) / SudocSudocFranceF

Early age behaviour of concrete nuclear containments

A numerical model has been developed to predict early-age cracking for massive concrete structures. Taking into account creep at early-age is essential if one wants to predict quantitatively the induced stresses if autogenous or thermal strains are restrained. Because creep strains may relax internal stresses, a creep model which includes the effects of hydration and temperature is used. For the prediction of cracking, a simple elastic damage model is used. Numerical simulations are performed in order to predict the behaviour of a massive wall and a concrete containment of a nuclear power plant. They show that significant relaxation of stresses (due to creep) occurs only after about 10 days, after cracking occurs. Moreover, since temperature in concrete may reach important values in massive concrete structures, it appears that effect of temperature on creep must be taken into account for an accurate prediction of cracking

Mechanical threshold of cementitious materials at early age

At early age, the mechanical characteristics of concrete, such as Young's modulus, follow a rapid rate of change. If strains are restricted or in the event of strain gradients, tensile stresses are generated and there is a risk of cracks occurring. Besides relaxation, change in Young's modulus as a function of the degree of hydration is a major parameter for the modeling of this phenomenon. In this evolution, a threshold of the degree of hydration has to be taken into account, below which concrete displays negligible stiffuess. For cement pastes, a simplified hydration model shows that percolation of the solid phases depends on the w/c ratio, which is in accordance with experimental results. On the other hand, for mortar or concrete, the presence of aggregates means that the solid volumetric fraction is such that percolation is observed before hydration occurs. Therefore another parameter is introduced: cohesion due to hydration products. By coupling our model with a finite-element code (CAST3M), it is shown that the threshold for Young's modulus in mortar is almost independent of the w/c ratio, which is in accordance with experimental results.Aujeune age, les caracteristiques mecaniques du beton, telles que le module d'Young, suivent une evolution rapide. Si les deformations sont genees, ou s 'if existe des gradients de deformation, des contraintes de traction sont generees et il y a un risque de fissuration. La variation du module d'Young enfonction du degre d'hydratation est un parametre majeur pour Ia modelisation de ce phenomene. Dans cette evolution, un seuil d'hydratation en de9a duquel Ia rigidite du beton est negligeable doit etre pris en compte. Pour des pates de ciment, un modele simplijie d'hydratation montre que la percolation des phases so/ides depend du rapport eau sur ciment, ce qui est en accord avec les resultats experimentaux. Pour les mortiers et les betons, Ia presence des granulats a pour consequence que Ia percolation de la fraction sol ide est observee meme pour un degre d'hydratation nul. C'est pourquoi nous introduisons un autre parametre : Ia cohesion due aux hydrates formes. En introduisant notre modele dans un code de calcul aux elements finis (CAST3M), nous montrons que le seuil d'hydratation du module d'Young du mortier est pratiquement independant du rapport eau sur ciment, ce qui est en accord avec les resultats experimentaux